Abstract
Researchers have recently focused on new and original roadside barriers that prioritize aesthetic, and environmental concerns by employing natural materials. In this study, the safety performance (Acceleration Severity Index (ASI), Theorical Head Impact Velocity (THIV)), structural performance (Working Width (W), Exit Angle (α)), and failure analysis (visual deformation) of a newly developed Renewable Hybrid Barrier (RHB) system at different timber thicknesses were tried to be determined by pendulum crash test and Finite Element (FE) models. The FE models were calibrated and validated based on pendulum crash test results, and then the most suitable timber thickness in terms of safety and structural performance was determined via FE analyses. The results revealed that as the timber thickness decreased, the safety parameters, such as ASI and THIV, decreased, thus the barrier safety increased. However, it was observed that the deflection and deformations in the barrier increased as the timber thickness decreased. In this sense, the safest and the most structurally durable barrier was determined through conducting virtual optimization tests. Studies on diversification of the usage areas of natural/renewable materials should be increased in the future.
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Renewable Hybrid Roadside Barrier: Optimization of Timber Thickness
Huseyin Yorur,a Sedat Ozcanan,b Halil Ibrahim Yumrutas,c,* and Emre Birinci d
Researchers have recently focused on new and original roadside barriers that prioritize aesthetic, and environmental concerns by employing natural materials. In this study, the safety performance (Acceleration Severity Index (ASI), Theorical Head Impact Velocity (THIV)), structural performance (Working Width (W), Exit Angle (α)), and failure analysis (visual deformation) of a newly developed Renewable Hybrid Barrier (RHB) system at different timber thicknesses were tried to be determined by pendulum crash test and Finite Element (FE) models. The FE models were calibrated and validated based on pendulum crash test results, and then the most suitable timber thickness in terms of safety and structural performance was determined via FE analyses. The results revealed that as the timber thickness decreased, the safety parameters, such as ASI and THIV, decreased, thus the barrier safety increased. However, it was observed that the deflection and deformations in the barrier increased as the timber thickness decreased. In this sense, the safest and the most structurally durable barrier was determined through conducting virtual optimization tests. Studies on diversification of the usage areas of natural/renewable materials should be increased in the future.
DOI: 10.15376/biores.18.1.804-826
Keywords: Timber; Wood material; Renewable material; Natural material; Wood mechanic; Numerical analysis
Contact information: a: Department of Wood Industry Engineering, Karabuk University, Box 78050, Karabuk, Turkey; b: Department of Civil Engineering, Sirnak University, Box 73000, Sirnak, Turkey; c: Department of Civil Engineering, Karabuk University, Box 78050, Karabuk, Turkey; d: Department of Forestry and Forest Products, Kastamonu University, Box 37000, Kastamonu, Turkey;
* Corresponding author: iyumrutas@karabuk.edu.tr
INTRODUCTION
One of the problems related to the increase in mobilization, especially in developing countries, is ensuring roadside traffic safety. To prevent or reduce the problems caused by traffic accidents, highway and traffic engineers make intense efforts and offer different solutions (Wright 1983; Roque et al. 2015; Ramiani and Shirazian 2020; Yumrutaş and Othman Ali 2022).
One of the solutions that serve this purpose is road restraint systems placed on the roadsides. There are various road restraint systems that are designated to redirect errant vehicles with a specified performance level and can provide guidance for risky/vulnerable roadsides such as sidewalks, bus stops, playgrounds, and petrol stations. These systems are guardrails, bollard systems, ditches, berms, fences, walls, planters, blockers, etc. They can be produced using different materials to serve different purposes. However, research studies have generally focused on the barrier types made of conventional materials such as concrete and steel. For this reason, steel guardrails, New Jersey concrete barriers, and cable barriers can be counted as barrier types that are generally accepted and frequently encountered in practice.
A few studies in the literature have focused on the use of only timber barriers or the reinforcement of timber barriers with steel or fiber additives (Lohrey et al. 1997; Ad 1999; Zhang et al. 2004, 2006; Bocchio et al. 2001; Leijten 2000, 2001; Davids et al. 2006; Kubojima et al. 2006; Watts and Morgan 2007; Faller et al. 2009; Marzougui et al. 2010; Wacker et al. 2010; Goubel et al. 2011; Cecháková et al. 2012; Pilia et al. 2012; Borovinšek et al. 2013; Bielenberg et al. 2014; Pilia and Maltinti 2014; Noda et al. 2016; van de Kuilen 2019).
Limited studies on new barrier types produced from innovative materials are available. Muller and Majerus (2002) explored the potential of recycled plastic bottles for use in roadside barriers and evaluated the effectiveness of the new barrier system employing computer software to reach the optimal solution. Amato et al. (2011) developed a new barrier type using low-cost building materials (stone and soil) to meet cost, aesthetic, and engineering requirements. The design consisted of linked steel gabions filled with stones, a type of structure generally used as a retaining wall but with different linkages. They observed the performance via real and virtual experiments. Anderson et al. (2012) added a flexible fabric material below the conventional guard rail and in front of the posts to provide a continuous protection system, especially for motorcyclists. They compared before and after crash statistics at a selected site and observed the contribution of the new material to existing guardrails in terms of performance. Atahan et al. (2002) manufactured guardrail posts from recycled materials and evaluated them for possible use in conventional steel guardrail systems. Static laboratory tests were conducted to determine the basic physical and mechanical properties of posts, and pendulum impact tests were performed to determine the dynamic response performance of the posts.
Recently, Yumrutaş et al. (2021) developed an innovative roadside barrier called “renewable hybrid barrier” (RHB) employing wood and sand material together. They conducted experimental pendulum crash tests and virtual tests to evaluate the efficiency of the novel barrier type. The results were successful for possible future novel barrier types. Yumrutaş and Ali (2022) developed renewable hybrid barriers using waste materials (slag and tyre). In this regard, they conducted pendulum crash tests to assess the performance considering the EN 1317 (2010) road restraint systems standard. The results indicated that hybrid barriers made of waste materials can be alternative to conventional barriers in terms of cost, environment, and aesthetics.
In this study, performance analyses for the normal (N2) containment level of wood material thickness in the renewable hybrid barrier (RHB) system, which was originally produced by Yumrutaş et al. (2021) using wood and sand, were revealed by virtual tests. In this context, verification and validation processes were completed based on the previous pendulum crash test data. After the model was verified, virtual full-scale tests based on the finite element method were applied for the TB 11 and TB 32 acceptance tests in line with the EN 1317 (2010) standard. The effects of timber thicknesses on the crash performance of RHBs were observed by considering the Acceleration Severity Index (ASI), Theoretical Head Impact Velocity (THIV), Working width (W), Exit angle (α)/exit box, and failure (visual deformation) parameters, and then the optimal timber thickness was determined.
EXPERIMENTAL
The Design of Renewable Hybrid Barrier
Renewable Hybrid Barriers (RHBs) comprise four main elements: timber, sand, concrete base, and steel frame. The concrete base is designed to separate timber materials from rainwater on the road platform. It is produced with C 30/37 grade cast in-situ concrete with the dimensions depicted in Fig. 1. The male and female parts enable interlocking with adjacent barriers, thus resisting impact energy together.
Fig. 1. Concrete base; all units are in mm
The steel frame that was made of S 235-grade rectangular section steel tube profile with 40 × 20 × 3 mm3 dimensions was used to fix timbers on it with screws and to give an F-shape form of RHBs (Fig. 2). They were inserted into the holes on the concrete base.
Fig. 2. Steel frame and the dimensions; all units are in mm
Wooden materials were produced from fir timbers (Abies nordmanniana subsp. equi-trojani) with thickness of 2, 4, 6, 8, and 10 cm. The half-lap method was applied to the joints of wooden timbers.
The main purpose of using sand in the design of RHBs is to benefit from the high energy absorption capacity of sand (Lambert et al. 2009; Sabet et al. 2009; Ho et al. 2013; Sy Ho and Masuya 2013; Bhatti 2015; Chian et al. 2017) and to diminish the cost. Crushed stone sand 0 to 5 mm in dimensions was not filled inside RHBs directly but first filled into the bags and then placed in the RHBs from the bottom level to the top. In this way, scattering pieces of the sand could be prevented after any accident, thus preventing a probable subsequent accident.
For each crash test, 3 RHBs were employed, and the finished product is presented in Figure 3 with the dimensions (Yumrutaş et al. 2021).
Fig. 3. The finished product of RHB; all units are in mm
Design and Concept of the Crash Pendulum System
The crash pendulum system includes a rigid frame, rammer, crane, and chains to suspend the rammer (Fig. 4). The pendulum rammer was lifted to a certain height with the help of a crane and released for free fall to generate the same energy for each experiment.
Fig. 4. Crash pendulum system (Yumrutaş et al. 2021)
The work of the pendulum system depends on the rule of preservation of energy. In position 1 the rammer has potential energy, and after release, it is converted to kinetic energy in position 2. Then it crashes into the barrier system with this energy. In this pendulum system, the rammer was lifted to 1.73 m height and generated 25.5 kJ energy for every test cycle (Yumrutaş et al. 2021).
RESEARCH METHODOLOGY
Research Process
There are various standards that can be used to assess the crash performance of vehicle barrier types, such as Road Restraint Systems standard EN 1317 (2010), Manual for Assessing Safety Hardware (MASH 2016), Recommended Procedures for the Safety Performance Evaluation of Highway Features (Ross et al. 1993), Roadside Design Guide (American Association of State Highway and Transportation Officials 2006), Standard test method for crash testing of vehicle security barriers (ASTM F2656 (2015), Impact test specifications for vehicle security barrier systems (BSI PAS 68 (2013), Vehicle security barriers (IWA 14 (2013), Road safety barrier systems (AS/NZS 3845 (1999), etc. In this study, EN 1317 (2010) Road Restraint Systems instructions were determined for optimization studies. The next step was the design of the RHBs and the design of the experimental crash pendulum system (Yumrutaş et al. 2021). The crash pendulum experiments provided verification data for virtual pendulum tests and a validation process for virtual full-scale crash tests was conducted. The last step was the optimization study to obtain the most suitable timber thickness resisting and absorbing the vehicle crash energy in terms of safety and structural performance, as well as cost. Figure 5 explains the research process step by step.
Fig. 5. Research process
Performance Metrics
The output assessment of the RHBs was controlled in accordance with EN 1317 (2010) Road Restraint Systems standard. This standard presents test techniques and impact test acceptance criteria that any barrier system needs to provide. The following performance criteria shall be met to restrain road vehicles (EN1317 (2010):
- The containment level;
- The acceleration severity index (ASI) and theoretical head impact velocity (THIV);
- The displacement as defined by the working width (W)
Road restraint systems shall meet the crash test requirements expressed in Table 1 and shall meet the requirements of Table 2 when tested following the impact test criteria presented in Table 1.
Table 1. Vehicle Impact Test Descriptions (EN1317 2010))
Table 2. Containment Levels (EN1317 (2010))
Because of their aesthetic view, there was a need for RHBs, particularly for scenic mountainous, touristic, and historical roadsides that have low traffic volume and are utilized predominantly by light vehicles. For this reason, TB 11 and TB 32 acceptance tests for N2 containment level were performed.
EN 1317 (2010) describes the ASI as a “measure of the severity of the motion for a person within a vehicle during an impact with a road restraint system” and the W as “the lateral distance between any part of the barrier on the not deformed traffic side and the maximum dynamic position of any part of the barrier.” The THIV idea was created to assess occupant impact severity for vehicles involved in crashes with barrier systems. These are basic qualities to demonstrate the productivity of road restraint systems. A barrier’s efficiency to keep the errant vehicle on the road platform decreases with the increased working width, so it should be within satisfactory cut-off points determined in standards. The ASI value is one of the signs of traveller security in the vehicle because of any impact with the barrier. The size of the speed of the THIV is a sign of the vehicle-to-restraint system impact severity. In this way, the ASI is determined by Eq. 1, and the THIV is determined by Eq. 2,
(1)
where 
x(t) is the maximum acceleration in x direction (m/s2), 
y(t) is the maximum acceleration in y direction (m/s2), and z(t) is the maximum acceleration in z direction (m/s2). Equation 2 for theoretical head impact velocity (THIV, km/s) is as follows,
(2)
Where Vx (t) is the impact velocity of the head along x direction (km/s), and Vy (t) is the impact velocity of the head along y direction (km/s).
The limit values of ASI and THIV safety parameters are given in Table 3. For assessing the safety performance of RHB, the following parameters were used.
Table 3. Impact Severity Levels (EN 1317 (2010))
Two metrics were used to observe structural performance. These are W and exit box/exit angle (α). Working width limits in EN 1317 (2010) are given in Table 4.
Table 4. Working Width Levels (EN 1317 (2010)
Exit box criteria shall be considered in virtual full-scale tests. The errant vehicle behavior after any accident shall be exceptionally critical and essential. An effective guardrail is supposed to put back the errant vehicle to pass over or supposed to redirect to the road platform at a small angle. In this way, a second probable car crash because of the errant vehicle that passes over the guardrail to the opposite lane or redirects back to the center of the initial traffic lanes can be prevented. The exit box can be determined with A and B values as shown in Table 5 and as illustrated in Fig. 6.
Table 5. Distance for Exit Box Criterion (EN 1317 (2010))
Fig. 6. The visual explanation of exit box concept (EN 1317 (2010))
Additionally, failure analysis needs to be observed by visual inspection. Finally, relative absolute error (RAE) metric was performed to verify and correctly control of the simulation results with the pendulum test results, taking into account Eq. 3,
(3)
where yp and ys are the pendulum result and the simulation result, respectively.
In this study, virtual tests were employed by experimentally verified data considering the performance metrics of related standards.
VIRTUAL TESTS
Roadside barriers (bollard systems) must undergo full-scale crash tests that have high costs and long processes to test their safety and structural performance. However, the performance of roadside barriers (bollard systems) can be simulated by finite element (FE) analyses more quickly, reliably, and economically than real tests (Apak 2019). The modelling and simulation of traffic accidents improved considerably, allowing researchers to model and analyze the performance of roadside barriers. Therefore, FE analyses have become an important tool to analyze roadside barriers with cost-effective solutions (Atahan and Cansiz 2005; Atahan 2006, 2010; Atahan et al. 2014). In this regard, LS-DYNA finite element simulation software that is used primarily in dynamic crashworthiness analysis was effectively applied in many studies (Wright and Ray 1996; Eskandarian et al. 1997; Ray 1997; Plaxico et al. 1998; Uddin and Hackett 1999; Atahan 2002, 2006; Tiso et al. 2002; Kirkpatrick et al. 2003; Bligh et al. 2004; Reid 2004; Whitworth et al. 2004; Wu and Thomson 2004; Griškevičius et al. 2007; Vesenjak et al. 2007; Tabacu and Pandrea 2008; Štych 2010; Ferdous et al. 2011; Amato et al. 2013; Borovinšek et al. 2013; Neuenhaus et al. 2013; Mirdamadi 2014; Mojdeh 2015; Teng et al. 2016; Yin et al. 2016; Soltani et al. 2017; Tabiei and Wu 2000, 2020; Baranowski and Damaziak 2021; Apak et al. 2022) and also will be applied in this study. Pendulum crash tests have also been conducted widely in the literature to verify and validate the virtual simulation tests (Gatchell and Michie 1974; Bank and Gentry 2001; Frp et al. 2001; Atahan et al. 2002; Mitchell et al. 2006; Atahan and Sevim 2008; Faller et al. 2009; Marzougui 2009; Ucar and Cengiz 2012; Kuilen 2019; Luo et al. 2020).
Verification Process of the Virtual Pendulum Test System
The experimental pendulum tests data used for verification was provided from the study by Yumrutaş et al. (2021). The wooden materials (timbers) of 2 and 4 cm in thickness were evaluated by crash pendulum tests. The results were employed for the verification process of virtual pendulum tests. Figure 7 shows the pendulum system and the FE models that were used for verification.
Fig. 7. (A) Pendulum system and (B) its FE model
The pendulum crash tests and virtual full-scale tests for TB 11 and TB 32 acceptance criteria were conducted considering the experiment matrix in Table 6.
Table 6. Experiment Matrix
Validation Process of Virtual Full-scale Crash Tests
Virtual full-scale tests were completed as TB 11 and TB 32 acceptance tests for N2 containment level using Ls-Dyna (Livermore Software Technology, Version R12.1.0, Livermore, CA, USA) simulation software. The sufficient barrier length was determined as 25 m to be able to observe the collaborative crash-resistant behavior of the RHBs and to observe the trajectory of the car on the road platform after any accident.
In this study, considering the TB 11 and TB 32 acceptance tests for N2 containment level, 900 kg, and 1500 kg in weight passenger car model that was validated by National Highway Traffic Safety Administration (NHTSA), was utilized as presented in Fig. 8 (NHTSA 2008). The original model was 900 kg, and the 1500 kg vehicle was adjusted to 1500 kg with the help of defined mass points (MassD) in the original model.
Fig. 8. The virtual passenger car validated by NHTSA 900 and 1500 kg in weight (NHTSA 2008)
The first grade (first quality) fir timbers were used. Consistent with the experimental data, input parameters of LS-DYNA were specified for the timber elements as follows. The moisture content of the timbers are % 8, bending strength is 72,92 N/mm2, modulus of elasticity is 9691,27 N/mm2, tensile strength parallel to fibers is 51.84 N/mm2, compressive strength parallel to fibers is 51.29 N/mm2, air dry density is 0.57 gr/cm3.
The exit box was determined as given in Table 7 and by utilizing the dimensions of the validated vehicle model, which was 3.75 m in length and 1.56 m in width (Fig. 9). According to the exit box, the maximum exit angle was calculated as approximately 24°, assuming that the crashed vehicle leaves within the short side of the exit box.
Fig. 9. The exit box boundaries for validated passenger car
RESULTS
The system had enough structural capacity to withstand the impact of the test vehicle and absorb the impact energy. In addition, the system also was able to contain and redirect the vehicle to the desired exit box boundaries. However, this study aimed to find the optimum timber thickness to attain the safest and the most cost-effective solution.
Real-Time/Virtual Pendulum Test Results
The real-time pendulum test results (Yumrutaş et al. 2021) were checked against the virtual pendulum model results for accurate verification.